In the U.S., are all voting booth areas required to have carbon monoxide detectors?

Basically, yes. Voting happens in churches, schools, and government buildings, which all have standard safety detectors. Furthermore, the fact voting is distributed across so many different kinds of locations means that it would be much harder for there to be a conspiracy to place faulty detectors in polling places.
- I agree with that second bit, but thinking of all the places I've voted in life I wouldn't imagine detectors in most of them. We hardly use gas at all around here, all electric, can't see how anything else would be a CO source.

I mean, voting booths aren't airtight, you could just have one for the polling place

In short, no.
Voting in the U.S. is run by the individual states, and each one sets their own rules and policies.
The federal government does set some minimum rules that only apply to federal elections, but those rules don't even require the use of voting booths: https://www.govinfo.gov/content/pkg/CFR-2024-title11-vol1/pdf/CFR-2024-title11-vol1.pdf
- Adding on to this; I'd be very surprised if there was a locality within the U.S. that didn't require every building to have carbon monoxide detectors, but again, voting doesn't even have to occur within a building.

I haven't voted in person for several years, but all the polling places I remember had all the doors open to the outside air. A basketball gym, a church side-hall, someone's home garage. And the booths are just curtained frames. But then again, I live in Los Angeles so it's not freezing in November. Maybe it's different in Minnesota.
- ...here we just have touchscreen kiosks set side-by-side along open tables, no privacy other than the LCD field-of-view...
  
  How odd! Everywhere I've voted in OK, IL and FL had pretty much the same setup, basically a 1/8th scale, standing office cubicle. You got me thinking, I'd be surprised to see anything else!

CO comes from incomplete combustion and you'd usually only have detectors for it around gas heaters, generators, stuff like that. Maybe you meant carbon dioxide (CO2). I don't remember ever seeing one around a voting booth. I'd consider them a good idea though, not because CO2 poisoning is a serious concern per se, but because high CO2 means that you're breathing air that other people exhaled, increasing your exposure to airborne pathogens.
- CO2 is a non-issue in open places with people around. The one gas our bodies detects well is CO2. If there's even a tiny percentage higher than background, people are going to quickly notice.
  
  In indoor crowded spaces, CO2 is often 2000 ppm or higher (background is now around 450). We might notice but just deal with it. In the past that meant getting sleepy at a lecture or that sort of thing, but today I'd consider it risky. I still wear an N95 mask whenever I'm in a public indoor space.

I believe any such regulations would be on a state by state basis, though I doubt any are actually enacted.